1. Field of the Invention
The present invention relates to a method and apparatus for hemming a workpiece such as a door panel and a hood panel for a vehicle.
2. Description of the Prior Art
Conventionally, as shown in FIGS. 8 and 9, peripheral edges, for example, of a door panel, a hood panel or the like for a vehicle are formed by hemming layers of an outer panel WO and an inner panel WI into an integral assembly. An example of a prior art apparatus to be used for such hemming is described in Japanese Laid-Open Patent Publication No. 61-262432 in which a roller type hemming apparatus is disclosed by the applicant of the present application.
The hemming apparatus includes a robot hand R (fragmentarily shown in FIGS. 8 and 9) having a predetermined traveling path which has been stored in advance and a hem roller r carried by the robot hand R, and is adapted to hem an upturned edge WE of a workpiece W by pressingly rotating the hem roller r along the upturned edge WE in such a manner as to accurately follow up the contour thereof. Thus, the hemming apparatus can carry out continuous hemming operation along the contour of the peripheral edge of the workpiece W, assuring satisfactory finish with improved hemming efficiency.
The outer panel WO has a peripheral edge preliminarily bent toward the inner panel WI substantially at right angles to form the above mentioned upturned edge WE (shown in a phantom line in FIG. 8) along which the hem roller r is pressingly rotated for hemming thereof. Such a hemming operation is generally carried out in two steps of a prebending process (FIG. 8) and a finish bending process (FIG. 9), and, in such a case, the hem roller r is controlled to rotate in such positions (i.e. tilt angles to the upturned edge WE) as to be suitable for the respective bending processes.
In the conventional prebending process, however, pressing force of the hem roller r is directly applied to the upturned edge WE, so that the upturned edge WE is bent sharply at its proximal end WE'. Such a sharp edge thus formed is disadvantageously left after finishing.